Linear optical scanners have application in surveillance systems. They are used in lens systems to periodically and reciprocally translate an image received by the system between a first and second limit position on a focal plane of the system. Prior linear optical scanners, as exemplified by U.S. Pat. No. 4,502,751 to Fjeldsted et al., are particularly adapted for use in light weight forward looking infrared surveillance systems.
The prior scanners have included first and second wedge prisms which are periodically and reciprocally counter-rotated through a predetermined angle about a common pivot axis. By counter-reciprocating the wedge prisms a light image passing through the prisms is translated between the first and second limit positions on a focal plane of the system.
One structure previously described for periodically and reciprocally counter-rotating each wedge prism comprises one or more cams in combination with cam followers and a return spring which holds the cam followers against the respective surfaces of the cam.
It has been found in certain applications of a linear optical scanner that it would be desirable to use a low-torque motor to drive the aforementioned cams. It is further desirable in many applications to reduce and minimize the weight and size of an optical scanner, as well as the power consumption of the drive motor. A need has also arisen for techniques to reduce the audible noise arising from the travel of the cam followers over the surfaces of cams and to reduce the radial force exerted on the cam shaft by the cam followers. It would also be highly desirable to maximize motor speed control for the drive motor of an optical scanner.
The return spring arrangement of prior devices for maintaining the cam followers against their respective cam surfaces prevents the above-noted desirable improvements from being implemented. The return spring of these prior devices is designed to provide at its point of minimum stretch, or most relaxed state, a predetermined minimum force to hold each cam follower against its respective cam surface. As the wedge-prisms are reciprocally counter-rotated, the return spring is alternately stretched and relaxed, thus producing an oscillating load on the cam followers. As the spring is increasingly stretched from its most relaxed state, a greater than the desired minimum load or force is exerted by the cam followers on the cam surface. This increased loading puts an undesirable excess radial force on the cam shaft and requires a higher power consumption by the drive motor than would be required if only the minimum desired force were exerted against the cam surfaces. This also results in increased audio noise generated at the interface between the cam followers and the cam surfaces, increased wear on the cam and cam followers, increased wear on the cam bearing and cam follower bearings, and an undesired increased radial loading on the cam drive gear head and outport bearings. Further, the oscillating load on the cams adversely affects motor speed control, particularly where a low-torque motor is used. In addition, the structure of the return spring arrangement of the prior devices requires greater weight and space than is optimum.
The Fjeldsted patent also describes another method of maintaining contact between the cam followers and the cams. In the alternately proposed method, a follower pin is inserted into a cam race or groove cut into a surface of the cam to describe the required cam travel of the follower while preventing the cam follower surfaces from leaving the cam surface. It has been found, however, that the restraints of manufacturing tolerances and the necessary allowance of sufficient clearance for the cam follower to travel smoothly in the groove result in an unacceptable lack of precision, for some applications, in the motion of the cam followers. Accordingly, it is necessary to load the cam follower with some force against one of the cam surfaces even if the cam follower travels in a groove in the cam.